Coolant

ABSTRACT

A refrigerant for a cooling device (10) comprising a cooling circuit (11) with at least one heat exchanger, in which the refrigerant undergoes a phase transition, the refrigerant being a refrigerant mixture composed of a mass fraction of carbon dioxide and a mass fraction of at least one other component, wherein the mass fraction of carbon dioxide in the refrigerant mixture is 10 to 50 mass percent, preferably 30 to 50 mass percent, the other component being pentafluoroethane and/or difluoromethane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application represents the national stage entry of PCTInternational Patent Application No. PCT/EP2018/072819 filed Aug. 23,2018, which claims priority to German Patent Application No. DE 10 2017120 786.4 filed Sep. 8, 2017 and German Patent Application No. DE 102017 216 363.1 filed Sep. 14, 2017. The contents of these applicationsare hereby incorporated by reference as if set forth in their entiretyherein.

The disclosure relates to a refrigerant for a cooling device and to atest chamber with the refrigerant and to a use of a refrigerant, therefrigerant for a cooling device which comprises a cooling circuit withat least one heat exchanger in which the refrigerant undergoes a phasetransition consisting of a refrigerant mixture composed of a massfraction of carbon dioxide and a mass fraction of at least one othercomponent.

Refrigerants of this kind typically circulate within a closed coolingcircuit of cooling devices and undergo a sequence of different changesin state of matter. Refrigerants should be of such a nature that theycan be used in a cooling circuit within a predefined temperaturedifference. Single-component refrigerants and refrigerant mixtures of atleast two components are known from the state of the art. Therefrigerants are classified according to DIN 8960 Section 6.

As per statutory regulations, a refrigerant must not significantlycontribute to the depletion of ozone in the atmosphere or to globalwarming. This means that essentially no fluorinated or chlorinatedsubstances can be used as refrigerants, which is why naturalrefrigerants or gasses are an option. Moreover, a refrigerant should benonflammable in order to not complicate filling, shipping and operationof a cooling circuit because of any safety regulations that may have tobe observed. Also, production of a cooling circuit becomes moreexpensive if a flammable refrigerant is used because of theconstructional measures required in that case. Flammability refers tothe refrigerant's property of reacting to ambient oxygen by releasingheat. A refrigerant is flammable in particular if it is classified infire class C of European standard EN2 and DIN 378 classes A2, A2L andA3.

Moreover, a refrigerant should have a relatively low CO₂ equivalent;i.e., a relative global warming potential (GWP) should be as low aspossible in order to avoid indirect damage to the environment in casethe refrigerant is released. The GWP indicates how much a defined massof a greenhouse gas contributes to global warming, carbon dioxideserving as the reference value. The value describes the mean warmingeffect over a specific period, 20 years being set here for the sake ofcomparability. For a definition of the relative CO₂ equivalent or GWP,reference is made to Intergovernmental Panel on Climate Change (IPCC),Assessment Report, Appendix 8.A, Table 8.A.1.

Refrigerants with a low GWP, such as <2500, have the disadvantage thatthese refrigerants tend to have a significantly lower cold capacity inthe temperature ranges relevant for a cooling circuit than refrigerantswith a comparatively higher GWP. A lower GWP can be achieved withrefrigerant mixtures that have a comparatively high mass fraction ofcarbon dioxide; however, these refrigerant mixtures may have zeotropicproperties due to the different substances mixed, which is undesirablein many cooling circuits.

In a zeotropic refrigerant mixture, a phase transition happens across atemperature range which is known as the temperature glide. Thetemperature glide refers to a difference between the boiling temperatureand the dew point temperature at constant pressure. Zeotropicrefrigerant mixtures typically contain a high mass fraction of anonflammable component, which is characterized by a comparatively highGWP, however. At first glance, carbon dioxide appears to be a suitablecomponent for a refrigerant mixture because it is nonflammable and has alow GWP. In a mixture of carbon dioxide with another component, however,it is essential that a mass fraction of a carbon dioxide has to becomparatively large if the other component is flammable. This isdisadvantageous, however, because carbon dioxide has a freezingtemperature or freezing point of −56.6° C., which hardly allowstemperatures of up to −60° C. to be achieved at a high carbon dioxideconcentration.

Also, the use of refrigerants should be as simple as possible, i.e., notrequire extensive technical restructuring of a cooling device. Withrefrigerants having a temperature glide of >3 K in particular, anexpansion element and a heat exchanger or evaporator of the coolingcircuit in question have to be adjusted to the evaporation temperatureof the refrigerant and corresponding control has to be provided.Furthermore, a distinction must be drawn between refrigerants that aredesigned for static operation of a cooling device, i.e., a coolingdevice having a temperature at the heat exchanger or evaporator that issubstantially constant over a longer period of time, and refrigerantsthat are designed for a dynamic cooling device, which exhibitscomparatively quick temperature changes at the heat exchanger. Dynamiccooling devices of this kind are integrated in test chambers, forexample, which means that a refrigerant used has to be usable within alarge temperature range.

Test chambers are typically used to test physical and/or chemicalproperties of objects, in particular devices. For instance, temperaturetest chambers or climate test chambers in which temperatures in a rangeof −60° C. to +180° C. can be set are known. In climate test chambers,desired climatic conditions can additionally be set, to which the deviceor the test material is then exposed for a defined period of time. Testchambers of this kind are often or sometimes realized as mobile deviceswhich are merely connected to a building via required supply lines andcomprise all modules needed to control the temperature and climate. Thetemperature of a test space holding the material to be tested istypically controlled in a circulating air duct within the test space.The circulating air duct forms an air treatment space in the test space,in which heat exchangers for heating or cooling the air flowing throughthe circulating air duct and the test space are disposed. A fan orventilator aspirates the air located in the test space and directs it tothe respective heat exchangers in the circulating air duct. In this way,the test material can be temperature-controlled or exposed to a definedtemperature change. During a test interval, a temperature can repeatedlychange between a maximum temperature and a minimum temperature of thetest chamber. A test chamber of this kind is known from EP 0 344 397 A2,for example.

The refrigerant circulating in a cooling circuit must be of such anature that it can be used in the cooling circuit within theaforementioned temperature difference. In particular, a dew pointtemperature of the refrigerant cannot be higher than a minimumtemperature of the temperature range of the cooling circuit that is tobe achieved because the minimum temperature would not be achievableotherwise when the refrigerant is evaporated in the heat exchangerserving to cool the test space. The dew point temperature of azeotropicrefrigerants is reached immediately behind the expansion element in theheat exchanger. Straight cooling circuits for test spaces require a veryhigh spatial temperature stability to precisely control the temperatureof the test chamber, which cannot be achieved at all or only to alimited degree using zeotropic refrigerants. High temperature stabilitycannot be achieved in this case because the dew point temperature or adew point of the zeotropic refrigerant may locally shift as a functionof a temperature in the test space in the area of the heat exchanger inthe test space because of temperature differences. Hence, a use ofzeotropic refrigerants, i.e., of refrigerants having a temperatureglide, in cooling circuits of test chambers is avoided.

Furthermore, cooling devices in which a zeotropic refrigerant mixture issuccessively evaporated are known. This means that components of therefrigerants are evaporated one after the other by means of an expansionelement. Cooling devices of this kind are also referred to as mixedfluid cascade systems and are suitable for realizing a substantiallystatic cryogenic temperature.

Hence, the object of the present disclosure is to propose a refrigerantfor a cooling device, a test chamber with a refrigerant, and a use of arefrigerant by means of which temperatures up to at least −60° C. can beachieved in an environmentally friendly and safe manner.

This object is attained by a refrigerant having the features of claim 1,a test chamber having the features of claim 12 and a use of arefrigerant having the features of claim 18.

In the refrigerant according to the disclosure for a cooling devicecomprising a cooling circuit with at least one heat exchanger, therefrigerant undergoes a phase transition in the heat exchanger, therefrigerant being a refrigerant mixture composed of a mass fraction ofcarbon dioxide and a mass fraction of at least one other component,wherein the mass fraction of carbon dioxide in the refrigerant mixtureis 10 to 50 mass percent, preferably 30 to 50 mass percent, the othercomponent being pentafluoroethane and/or difluoromethane.

Carbon dioxide (CO₂) is also known as a refrigerant or component underthe designation R744, pentafluoroethane (C₂HF₅) is known under thedesignation R125 and difluoromethane (CH₂F₂) is known under thedesignation R32, trifluoroethylene is known under the designation R1123,1,1-difluoroethene (C₂H₂F₂) is known under the designation R1132a, andfluoroethene (C₂H₃F) is known under the designation R1141 according tothe latest version of German industry standard DIN 8960 as prior to thepriority date of the application.

The disclosure provides a refrigerant mixture of carbon dioxide and oneor more fluorinated refrigerants which have a low GWP and arenonflammable or flammable to a limited degree only. A fraction of carbondioxide has to be as low as possible because otherwise a freezing pointof the refrigerant mixture would rise with an increasing mass fractionof carbon dioxide. However, a lower mass fraction of carbon dioxidereduces a GWP-reducing effect of the carbon dioxide. This is why partlyfluorinated refrigerants have a significantly higher GWP than carbondioxide, while also having an improved flame-retardant effect.Pentafluoroethane and difluoromethane in particular contain significantamounts of fluorine atoms, which leads to an undesirably high GWP. Aswas surprisingly found, however, a sufficiently low GWP, i.e., <150, forexample, can be achieved with a refrigerant mixture containing a massfraction of carbon dioxide of 30 to 40 mass percent withpentafluoroethane and/or difluoromethane. As was also found, aflame-retardant effect of pentafluoroethane is comparatively greaterthan that of carbon dioxide. Moreover, the negative properties ofpentafluoroethane and of carbon dioxide can be reduced by addingdifluoromethane as a third component of the refrigerant mixture. Thus, arefrigerant mixture containing pentafluoroethane and difluoromethane canbe classified as nonflammable. At the same time, difluoromethane has alower freezing temperature with carbon dioxide than withpentafluoroethane. Consequently, a mixture of pentafluoroethane,difluoromethane and carbon dioxide can achieve a lower freezingtemperature than pentafluoroethane and carbon dioxide alone.Difluoromethane thus lowers the freezing point of the refrigerantmixture significantly, a certain mass fraction of carbon dioxide beingrequired in order for the refrigerant mixture to be nonflammable. At thesame time, however, difluoromethane leads to a high final compressiontemperature, which is why difluoromethane is suitable only within limitsas a sole mixing partner for carbon dioxide. Pentafluoroethane cannotlower a freezing point of the refrigerant mixture as far asdifluoromethane, but has a greater flame-retardant effect than carbondioxide, which is advantageous.

The carbon dioxide is mixable with pentafluoroethane and/ordifluoromethane in a particularly advantageous manner if the massfraction of carbon dioxide in the refrigerant mixture is 31 to 46 masspercent. A freezing point of the refrigerant mixture can be reduced byadding the mentioned components. This reduction can be set in such amanner that the freezing point of the refrigerant mixture is lower thanthe intended evaporation temperature and, at the same time, the vaporpressure associated with the evaporation temperature may be higher oronly slightly lower than the ambient pressure.

In an embodiment of the refrigerant, a mass fraction of carbon dioxidemay be 30 to 33 mass percent, preferably 31 mass percent, and a massfraction of pentafluoroethane may be 70 to 67 mass percent, preferably69 mass percent. Thus, a refrigerant mixture consisting solely of carbondioxide and pentafluoroethane may be produced in this case.Pentafluoroethane is nonflammable, which means that all mixturescontaining pentafluoroethane and carbon dioxide are nonflammable. Thefreezing point is not reduced as far compared to difluoromethane andR1123. Its GWP of 3150 is significantly higher than that of otherpossible components. Hence, it may also be partly replaced with othersubstances in the refrigerant mixture in order to reduce the GWP of therefrigerant mixture. The flame-retardant effect of pentafluoroethane isgreater than that of carbon dioxide, which means that a mass fraction ofcarbon dioxide in the refrigerant mixture can be reduced, which lowersthe freezing point further and still ensures non-flammability butincreases the GWP. Thus, mixtures of carbon dioxide, pentafluoroethane,and R1132A, R1123 and R1141 are also possible.

In another embodiment of the refrigerant, a mass fraction of carbondioxide may be 44 to 48 mass percent, preferably 45.8 mass percent, andthe mass fraction of difluoromethane may be 56 to 52 mass percent,preferably 54.2 mass percent. In this case, the refrigerant mixture mayconsist solely of carbon dioxide and difluoromethane. A mass fraction ofcarbon dioxide of at least 35 mass percent is sufficient for therefrigerant mixture to be classified as nonflammable. The freezing pointis reduced further than with R125, R1132A and R1141, which is why areduction of the freezing point can be made possible However, a GWP ofdifluoromethane is greater than that of R1132A, R1141, R1123 and carbondioxide. Difluoromethane can be called a small molecule, which has theconsequence that the final compression temperature of difluoromethane ishigher than that of larger and heavier molecules such aspentafluoroethane when compressed under the same technical conditions.Refrigerants R410A and R410B exhibit lower final compressiontemperatures than difluoromethane, which is why they are particularlysuitable mixing partners with carbon dioxide.

Consequently, the refrigerant mixture may be a binary mixture.

The refrigerant mixture may have trifluoroethylene, 1,1-difluoroetheneor fluoroethene as an additional component. 1,1-Difluoroethene has avery low GWP, making it a suitable component for reducing the GWP.Furthermore, it has a very low freezing point and a very low normalboiling point, which makes it usable for cryogenic applications.However, 1,1-difluoroethene and fluoroethene are flammable and, ifadmixed, should not amount to more than 10 m % in the refrigerantmixture. Trifluoroethylene has a very low GWP, making it a suitablecomponent for reducing the GWP. Since it is thermodynamically similar todifluoromethane, refrigerant mixtures of carbon dioxide, difluoromethaneand trifluoroethylene can be produced. Similar to difluoromethane,trifluoroethylene may lead to a great reduction of the freezing point(greater than pentafluoroethane). Mass fractions of trifluoroethyleneand difluoromethane may preferably be 45/55.

In another embodiment, the refrigerant may have a mass fraction ofcarbon dioxide of 33 to 38 mass percent, preferably 35 mass percent, amass fraction of pentafluoroethane of 33.5 to 31 mass percent,preferably 32.5 mass percent, and a mass fraction of difluoromethane of33.5 to 31 mass percent, preferably 32.5 mass percent. Accordingly, therefrigerant mixture may consist of no more than three components. Themass fractions of the components difluoromethane and pentafluoroethaneare equal in that case. As described above, a mixture of carbon dioxidewith pentafluoroethane and difluoromethane has proven particularlyadvantageous. This refrigerant mixture may have a temperature glideof >7 K at evaporation pressures around 1 bar. Furthermore, thisrefrigerant mixture leads to a concentration-dependent reduction of thefreezing point. Hence, flammable and nonflammable refrigerant mixturesfor different temperature applications may arise when mass fractionsdeviate from the indicated mass fractions.

In another embodiment, a mass fraction of carbon dioxide in therefrigerant may be 10 to 35 mass percent, in which case a mass fractionof pentafluoroethane and a mass fraction of difluoromethane together are65 to 80 mass percent and a ratio of the mass fractions ofpentafluoroethane and difluoromethane may be 0.7 to 1.3.

Furthermore, a mass fraction of carbon dioxide may be 33 to 38 masspercent, preferably 35 mass percent, and a mass fraction of refrigerantR410A or refrigerant R410B may be 67 to 62 mass percent, preferably 65mass percent. Refrigerant R410A contains equal mass fractions ofpentafluoroethane and difluoromethane. Refrigerant R410A is a readymaderefrigerant mixture easily available for purchase, which means that therefrigerant can be prepared in a cost-effective and simple manner bysimply mixing carbon dioxide with R410A.

In the following Table, examples of refrigerants according to theembodiments described above are indicated.

TABLE Refrigerant 1 2 3 4 R744 31 45.8 35 35 (wt %) R125 69 32.5 (wt %)R32 54.2 32.5 (wt %) R410A 65 (wt %) Temperature 17.1 15.9 15.7 15.7glide absolute (K) Temperature −77.7 to −78.7 to −77 to −77 to glide−60.6 −62.8 −61.3 −61.3 (° C.) Freezing −80 −87 −86 −86 point (° C.)

Furthermore, the refrigerant may have a temperature glide of ≥10 K,preferably ≥15 K, particularly preferably ≥18 K. A temperature glide ofthe refrigerant should not be >20 K so that a cooling device can beoperated in a reasonable way.

The refrigerant may have a relative CO₂ equivalent of <2500, preferably<1500, particularly preferably <500, over 20 years. Consequently, therefrigerant may be of little harm to the environment.

The refrigerant may be nonflammable. If the refrigerant is nonflammable,the cooling circuit and a test chamber in particular can be designedmore cost-effectively because no special safety measures in terms offlammability of the refrigerant will have to be observed. In this case,the refrigerant may at least not be classified in fire class C and/orrefrigerant safety group A1. Moreover, shipping and transport of thecooling circuit is easier because the cooling circuit can be filled withthe refrigerant before being transported, irrespective of the mode oftransport. If a flammable refrigerant is used, filling may not bepossible until start-up at the installation site. Furthermore, use ofthe nonflammable refrigerant in the presence of ignition sources ispossible.

The test chamber according to the disclosure for conditioning aircomprises a test space which serves to receive test material and whichcan be sealed against an environment and is temperature-insulated, and atemperature control device for controlling the temperature of the testspace, a temperature in a temperature range of −60° C. to +180° C. beingestablishable within the test space by means of the temperature controldevice, the temperature control device having a cooling devicecomprising a cooling circuit with a refrigerant according to thedisclosure, a heat exchanger, a compressor, a condenser and an expansionelement. Regarding the advantages of the test chamber according to thedisclosure, reference is made to the description of advantages of therefrigerant according to the disclosure.

By means of the temperature control device, a temperature in atemperature range of −60° C. to +180° C., preferably −80° C. to +180°C., particularly preferably −100° C. to +180° C., may be establishedwithin the test space. Unlike in a mixed fluid cascade system, therefrigerant with all components contained in the refrigerant can beevaporated at once by means of the expansion element. Since a freezingpoint of the carbon dioxide is −56.6° C., refrigerant mixtures thatcontain a large mass fraction of carbon dioxide are no longer suitablefor achieving temperatures below −56.6° C., on principle. The use of therefrigerant according to the disclosure, however, allows a dew pointtemperature of the refrigerant of less than −60° C. to be achieved.

The cooling circuit may have an internal heat exchanger, and theinternal heat exchanger may be connected to a high-pressure side of thecooling circuit upstream of the expansion element and downstream of thecondenser and to a low-pressure side of the cooling circuit upstream ofthe compressor and downstream of the heat exchanger. By use of theinternal heat exchanger and cooling of the liquefied refrigerant of thehigh-pressure side by means of the internal heat exchanger, temperaturesbelow −56° C. can be reached easily. The evaporation temperature of therefrigerant cooled by means of the internal heat exchanger can bereduced at the expansion element relative to an evaporation temperatureof an uncooled refrigerant. The cold capacity transferred from thelow-pressure side to the high-pressure side via the internal heatexchanger can thus be used at least in part, preferably exclusively, toreduce the evaporation temperature of the refrigerant at the expansionelement. Furthermore, use of a zeotropic refrigerant having atemperature glide is made possible in the first place because thelocation of the dew point temperature of the refrigerant or the dewpoint of the refrigerant can be shifted into the internal heat exchangerin this case. As a consequence of the temperature glide of the zeotropicrefrigerant, the achieved dew point temperature of the refrigerant maybe comparatively high and thus prevent the heat exchanger from coolingfurther.

Hence, only part of the refrigerant may be evaporated in the heatexchanger and the unusable part of the wet vapor portion of therefrigerant can be shifted into the internal heat exchanger. On thewhole, this allows refrigerants which contain a mass fraction of carbondioxide and which, while being environmentally friendly, have zeotropicproperties to be used to establish low temperatures in a test space.Moreover, if part of the temperature glide or part of the wet vapor ofthe refrigerant is shifted from the heat exchanger in the test spaceinto the internal heat exchanger, a comparatively improved temperaturestability can be achieved with the zeotropic refrigerant. In this case,a cold capacity output via the heat exchanger can be generated in asection of the temperature glide only, which means that a shift of thedew point of the refrigerant in the cooling circuit has hardly anyimpact on a temperature stability of the heat exchanger. Furthermore, asingle heat exchanger may be used to cool a fluid, i.e., the air in thetest space, in this case.

The heat exchanger may be dimensioned in such a manner that only part ofthe refrigerant can evaporate in the heat exchanger. This results in theadvantage that the dew point or the location of the dew pointtemperature of the refrigerant can be shifted out of the heat exchangerinto the internal heat exchanger. Because of a temperature glide of thezeotropic refrigerant, partial evaporation of the refrigerant in theheat exchanger achieves a lower temperature in the heat exchanger thanthe following remaining evaporation of the refrigerant in the internalheat exchanger.

In one embodiment of the test chamber, the heat exchanger may bedisposed in the test space. In this case, the heat exchanger may also bedisposed in an air treatment space of the test space so that aircirculated by a fan can come into contact with a heat exchanger. In thisway, a circulated amount of air of the test space can be cooled directlyin the test space by means the cooling device via the heat exchanger.The test chamber may have the cooling circuit as a sole, single coolingcircuit. In this case, the cooling circuit is connected directly to thetest space.

In another embodiment of the test chamber, the condenser may be realizedas a cascade heat exchanger of another cooling circuit of the coolingdevice. Accordingly, the test chamber may have at least two coolingcircuits, in which case the cooling circuit may form a second stage ofthe cooling device and another cooling circuit, which is disposedupstream of the cooling circuit, may form a first stage of the coolingdevice. In this case, the condenser serves as a cascade heat exchangeror a heat exchanger for the cooling circuit. This embodiment of a testchamber allows particularly low temperatures to be established in thetest space.

The temperature control device may have a heating device comprising aheater and a heating heat exchanger in the test space. The heatingdevice may be an electric resistance heater which heats the heating heatexchanger in such a manner that the temperature in the test space can beraised by means of the heating heat exchanger. If the heat exchanger andthe heating heat exchanger can be specifically controlled by means of acontrol device to cool or heat the air circulated in the test space, atemperature in the temperature range indicated above can be establishedwithin the test space by means of the temperature control device. Atemperature stability over time of ±1 K, preferably ±0.3 K to ±0.5 K orless than ±0.3 K, may be established in the test space during a testinterval irrespective of the test material or of an operating state ofthe test material. A test interval is a segment of a full test period inwhich the test material is exposed to a substantially constanttemperature or climatic condition. The heating heat exchanger may becombined with the heat exchanger of the cooling circuit in such a mannerthat a shared heat exchanger body through which the refrigerant can flowand which has heating elements of an electric resistance heater can berealized. The condenser may be cooled with air, water or anothercoolant. In principle, the condenser can be cooled using any suitablefluid. The essential aspect is that the thermal load generated at thecondenser is discharged via the cooling air or the cooling water in sucha manner that the refrigerant can condense until it is completelyliquefied.

A first bypass having at least one controllable second expansion elementmay be realized in the cooling circuit, in which case the first bypassmay be connected to the cooling circuit upstream of the internal heatexchanger and downstream of the condenser and the first bypass may berealized as a controllable additional internal cooling system. The firstbypass may thus form a re-injection device for refrigerant. Accordingly,refrigerant can be recycled from the controllable second expansionelement in the internal heat exchanger on the low-pressure side. In thiscase, the first bypass may be connected to the low-pressure side of thecooling circuit upstream of the internal heat exchanger and downstreamof the heat exchanger. The refrigerant cooled or having its temperaturelevel lowered by the second expansion element may be led through theinternal heat exchanger and intensify cooling of the refrigerant on thehigh-pressure side of the internal heat exchanger. Also, a coolingcapacity of the internal heat exchanger can be controlled even moreprecisely in this way.

A second bypass comprising at least one third expansion element may beformed in the cooling circuit, in which case the second bypass bypassesthe expansion element downstream of the condenser and upstream of theinternal heat exchanger and the refrigerant can be metered by means ofthe third expansion element in such a manner that a suction gastemperature and/or a suction gas pressure of the refrigerant can becontrolled upstream of the compressor on the low-pressure side of thecooling circuit. In this way, potential overheating and damage of thecompressor, which may be a compressor device, for example, can beprevented among other things. Consequently, gaseous refrigerant locatedupstream of the compressor can be cooled via the second bypass byactuation of the third expansion element by adding still-liquidrefrigerant. The third expansion element can be actuated by means of acontrol device which itself is coupled to a pressure and/or temperaturesensor in a cooling circuit upstream of the compressor. Particularlyadvantageously, a suction gas temperature of ≤30° C. can be set via thesecond bypass. Also, the refrigerant can be metered in such a mannerthat an operating time of the compressor can be controlled. Onprinciple, it is disadvantageous for the compressor or compressor deviceto be switched on and off repeatedly. A service life of a compressor canbe prolonged if the compressor operates for longer periods of time. Arefrigerant can be led past the expansion element or the condenser viathe second bypass in order to delay an automated deactivation of thecompressor and to prolong an operating time of the compressor, forexample.

Another bypass comprising at least one other expansion element may beformed in the cooling circuit, the other bypass bypassing the compressordownstream of the compressor and upstream of the condenser in such amanner that a suction gas temperature and/or a suction gas pressure ofthe refrigerant can be controlled upstream of the compressor on thelow-pressure side of the cooling circuit and/or that a pressuredifference between the high-pressure side and a low-pressure side of thecooling circuit can be equalized. The second bypass may additionally beequipped with a settable or controllable valve, such as a magneticvalve. Connecting the high-pressure side and the low-pressure side viathe other expansion element ensures that the gaseous refrigerant thuscompressed gradually flows from the high-pressure side to thelow-pressure side of the cooling circuit in the event of a systemstandstill. This also ensures gradual pressure equalization between thehigh-pressure side and the low-pressure side even when the expansionelement is closed. A cross-section of the other expansion element may bedimensioned in such a manner that the refrigerant flowing from thehigh-pressure side to the low-pressure side has only a marginal impacton the normal operation of the cooling device. At the same time, agaseous refrigerant located upstream of the compressor may be cooled byadding the liquid refrigerant via the other bypass.

Furthermore, the internal heat exchanger may be realized as asub-cooling section or a heat exchanger, in particular a plate heatexchanger. The sub-cooling section may simply be realized by two linesections of the cooling circuit that are in contact with each other.

The expansion element may have a throttle and a magnetic valve, in whichcase refrigerant can be metered via the throttle and the magnetic valve.The throttle may be a settable valve or a capillary via whichrefrigerant is routed by means of the magnetic valve. The magnetic valveitself may be actuated by means of a control device.

Also, the temperature control device may comprise a control devicecomprising at least one pressure sensor and/or at least one temperaturesensor in the cooling circuit, in which case a magnetic valve can beactuated by means of the control device as a function of a measuredtemperature and/or pressure. The control device may comprise means fordata processing which process sets of data from sensors and control themagnetic valves. In this case, a function of the cooling device may alsobe adjusted to the refrigerant used via an appropriate computer program,for example. Furthermore, the control device may signal a malfunctionand initiate a shut-down of the test chamber, if necessary, in order toprotect the test chamber and the test material from damage due tocritical or undesirable operating states of the test chamber.

When a refrigerant consisting of a refrigerant mixture composed of amass fraction of carbon dioxide of 10 to 50 mass percent, preferably 30to 50 mass percent, and a mass fraction of at least one other component,the other component being pentafluoroethane and/or difluoromethane, isused according to the disclosure, the refrigerant is used to conditionair in a test space of a test chamber, the test space serving to receivetest material and being sealable against an environment andtemperature-insulated, a cooling device of a temperature control deviceof the test chamber comprising a cooling circuit with the refrigerant, aheat exchanger, a compressor, a condenser and an expansion element isused to establish a temperature in a temperature range of −60° C. to+180° C., preferably −70° C. to +180° C., particularly preferably −80°C. to +180° C., within the test space.

The refrigerant can be cooled by means of an internal heat exchanger ofthe cooling circuit, which is connected to a high-pressure side of thecooling circuit upstream of the expansion element and downstream of thecondenser and to a low-pressure side of the cooling circuit upstream ofthe compressor and downstream of the heat exchanger, of thehigh-pressure side, the cooling of the refrigerant of the high-pressureside by means of the internal heat exchanger being usable to lower anevaporation temperature at the expansion element. During lowering of theevaporation temperature of the refrigerant of the high-pressure side, asuction pressure of the refrigerant of the low-pressure side can be keptconstant. A greater system complexity, such as in the form of additionalcontrol of the suction pressure and control of the expansion element asa function of the suction pressure, is not necessarily required in thatcase. In particular, the compressor may also be operated at constantoutput irrespective of an operating state of the cooling circuit. Whenpiston pumps are used as compressors in particular, it is essential forthem to be in operation for long periods of time and at a constant speedin order to achieve a long service live.

The refrigerant of the high-pressure side may be cooled by therefrigerant of the low-pressure side at a constant suction pressure onthe low-pressure side by means of the internal heat exchanger.Consequently, the refrigerant can evaporate at constant suction pressureon an evaporation section of the cooling circuit from the expansionelement up to and including the internal heat exchanger. If the suctionpressure or evaporation pressure of the refrigerant is constant, therefrigerant can evaporate from the expansion element at a lowevaporation temperature to the internal heat exchanger at a highevaporation temperature according to the temperature glide of therefrigerant. The dew point temperature resulting from the temperatureglide may be higher than the temperature of the fluid to be cooled or ofthe air in the test space. Once an evaporation temperature of therefrigerant is equal to the temperature of the air to be cooled in thetest space at the same suction pressure, the air cannot be cooled anyfurther. However, the dew point temperature reached in the other heatexchanger is lower than the liquid temperature of the refrigerant on thehigh-pressure side of the internal heat exchanger, which means that aliquid temperature of the refrigerant can be reduced further.Accordingly, an evaporation temperature downstream of the expansionelement can be lowered without changing the suction pressure, allowingfurther cooling of the air in the test space to be achieved.

Thus, a first portion of the refrigerant routed via the expansionelement can be evaporated in the heat exchanger and a second portion ofthe refrigerant can be evaporated in the internal heat exchanger. Anevaporation section of the cooling circuit within which the refrigerantevaporates may extend from the expansion element as far as to theinternal heat exchanger. The evaporation section may run through theinternal heat exchanger, in which case a dew point of the refrigerantmay be located at an exit of the internal heat exchanger upstream of thecompressor. A first portion/second portion ratio may change duringoperation of the cooling circuit as a function of a temperature in thetest space or at the heat exchanger. For example, a comparatively largetemperature difference between the temperature of the heat exchanger anda temperature in the test space may lead to accelerated heating of therefrigerant in the heat exchanger, which results in a shift of the dewpoint of the refrigerant toward an entry of the internal heat exchangeror an exit of the heat exchanger upstream of the compressor. This kindof shift of the dew point can be tolerated as long as no comparativelylow temperature or target temperature has been established in the testspace yet. When the temperature of the heat exchanger approaches thetemperature in the test space, the dew point shifts and the secondportion thus grows relative to the first portion of the refrigerant.

The evaporation temperature of the refrigerant of the high-pressure sidecan be lowered in a self-controlled manner. Depending on the temperatureat the heat exchanger, refrigerant no longer evaporating can bedischarged from the heat exchanger in the flow direction because thetemperature at the heat exchanger is no longer sufficient to cause aphase transition of the refrigerant in this case. Thus, wet vapor orliquid refrigerant is re-evaporated in the internal heat exchangerbecause here a temperature difference between the high-pressure side andthe low-pressure side can always be greater than at the heat exchanger.If a temperature of the liquid refrigerant upstream of the expansionelement is reduced by means of the internal heat exchanger by the heatexchange at the internal heat exchanger, the energy density of therefrigerant upstream of the expansion element and the temperaturedifference thus achievable at the heat exchanger increase. Theinteraction of the expansion element, the heat exchanger and theinternal heat exchanger does not have to be controlled, in principle.

Particularly advantageously, the cooling device is operated exclusivelybelow the critical point of the refrigerant. If the cooling device isoperated below the triple point of the refrigerant, reaching of asupercritical state of the refrigerant can be precluded. Thus, thecooling device does not have to be configured for operation in thesupercritical state, which saves costs for production of the coolingdevice.

In particular, the constant suction pressure may also be maintainedduring lowering of the evaporation temperature of the refrigerant of thehigh-pressure side by means of the internal heat exchanger. Accordingly,the cooling of the refrigerant of the high-pressure side via theinternal heat exchanger can also be exploited in part or exclusively tolower an evaporation temperature of the refrigerant at the expansionelement.

A dew point temperature of the refrigerant may be higher than a minimumtemperature of the temperature range. In the test chambers known fromthe state of the art, the minimum temperature of the temperature rangecan no longer be established with a refrigerant of this kind in thatcase, but a comparatively higher minimum temperature, whichsubstantially corresponds to the dew point temperature of therefrigerant. In the test chamber according to the disclosure, however, arefrigerant whose dew point temperature is higher than an achievableminimum temperature of the temperature range can be used because theliquefied refrigerant on the high-pressure side can be cooled by meansof the internal heat exchanger, which means that an evaporationtemperature of the refrigerant at the expansion element can becomparatively lower.

The refrigerant can be evaporated absolutely at a suction pressure orevaporation pressure in a pressure range of 0.3 to 5 bar. Use of therefrigerant within that pressure range allows cost-effective productionof the cooling circuit because no special pressure-resistant modules andcomponents have to be used to construct the low-pressure side of thecooling circuit.

Also, the refrigerant can be condensed absolutely at a condensationpressure in a pressure range of 5 to 35 bar. Here, too, thehigh-pressure side can be constructed using modules and components thatdo not have to be adapted to comparatively higher pressures.

Other embodiments of a use are apparent from the description of featuresof the claims depending on device claim 1.

Hereinafter, preferred embodiments of the disclosure will be explainedin more detail with reference to the accompanying drawings.

FIG. 1 is a pressure-enthalpy diagram for a refrigerant;

FIG. 2 is a schematic illustration of a first embodiment of a coolingdevice;

FIG. 3 is a schematic illustration of a second embodiment of a coolingdevice;

FIG. 4 is a schematic illustration of a third embodiment of a coolingdevice;

FIG. 5 is a schematic illustration of a fourth embodiment of a coolingdevice;

FIG. 6 is a schematic illustration of a fifth embodiment of a coolingdevice.

FIG. 2 shows a first embodiment of a cooling device 10 of a test chamber(not shown). Cooling device 10 comprises a cooling circuit 11 with arefrigerant, a heat exchanger 12, a compressor 13, a condenser 14 and anexpansion element 15. Condenser 14 is cooled by another cooling circuit16 in the case at hand. Heat exchanger 12 is disposed in a test space(not shown) of the test chamber. Furthermore, cooling circuit 11 has ahigh-pressure side 17 and a low-pressure side 18, to which an internalheat exchanger 19 is connected.

FIG. 1 shows a pressure-enthalpy diagram (log p/h diagram) for therefrigerant circulating in cooling circuit 11, the refrigerant being azeotropic refrigerant. According to a combined view of FIGS. 1 and 2,starting from position A, the refrigerant upstream of compressor 13 isaspirated and compressed, whereby a pressure is achieved downstream ofcompressor 13 according to position B. The refrigerant is compressed bymeans of compressor 13 and is subsequently liquefied in condenser 14according to position C. The refrigerant passes through internal heatexchanger 19 on high-pressure side 17, where it is cooled further,position C′ upstream of expansion element 15 thus being reached. Bymeans of internal heat exchanger 19, the portion of the wet vapor area(positons E to E′) not usable in heat exchanger 12 can be used tofurther reduce a temperature of the refrigerant (positions C′ to C). Atexpansion element 15, the refrigerant is relaxed (positions C′ to D′)and partially liquefied in heat exchanger 12 (positions D′ to E). Then,the wet vapor of the refrigerant enters internal heat exchanger 19 onlow-pressure side 18, where the refrigerant is re-evaporated until thedew-point temperature or the dew point of the refrigerant is reached atposition E′. Hence, a first subsection 20 of an evaporation section 22of the refrigerant runs through heat exchanger 12, a second subsection21 of evaporation section 22 running through internal heat exchanger 19.The essential aspect is that a suction pressure of compressor 13 onlow-pressure side 18 is kept constant on evaporation section 22 even ifthe evaporation temperature at expansion element 15 changes.

The refrigerant is a refrigerant mixture composed of a mass fraction ofcarbon dioxide of 30 to 50 mass percent and a mass fraction of at leastone other component, the other component being pentafluoroethane and/ordifluoromethane. In principle, it is possible for the refrigerantslisted in the Table above to be used in cooling circuit 11 and in thecooling circuits described below.

FIG. 3 shows a schematic illustration of a simplest embodiment of acooling device 23, cooling device 23 being self-controlling. Coolingdevice 23 comprises a cooling circuit 24 with a heat exchanger 25, acompressor 26, a condenser 27, an expansion element 28 and an internalheat exchanger 29. Depending on a temperature at heat exchanger 25,refrigerant not fully evaporated escapes from heat exchanger 25 becausethe temperature at heat exchanger 25 or in a test space (not shown) isno longer high enough to cause a phase transition. In this case,refrigerant still liquid is re-evaporated in internal heat exchanger 29because a temperature difference there has to be greater than at heatexchanger 25 at all times. Once the temperature of the liquidrefrigerant upstream of expansion element 28 has been reduced by heatexchange in internal heat exchanger 29, the energy density and thetemperature difference achievable with it at heat exchanger 25 increase.Cooling device 23 does not require elaborate control by way of sensorsetc.

FIG. 4 shows a cooling device 30 which differs from the cooling deviceof FIG. 3 in that it has a first bypass 31 and a second bypass 32. Acontrollable second expansion element 33 is disposed in first bypass 31,first bypass 31 being configured as an additional internal coolingsystem 34. First bypass 31 is connected to cooling circuit 24immediately downstream of condenser 27 upstream of internal heatexchanger 29 and downstream of heat exchanger 25 and upstream ofinternal heat exchanger 29. First bypass 31 thus bypasses expansionelement 28 with heat exchanger 25, internal heat exchanger 29 beingsuppliable with evaporating refrigerant via second expansion element 33.A suction gas mass flow introduced into internal heat exchanger 29 canbe cooled additionally by means of first bypass 31 in case of highsuction gas temperatures, which may be caused by heat exchanger 25. Inthis way, evaporation of refrigerant upstream of the expansion elementcan be precluded. Hence, first bypass 31 can be used to react tochanging load cases of cooling device 30. Second bypass 32 has a thirdexpansion element 35 and is connected to cooling circuit 24 downstreamof condenser 27 and upstream of internal heat exchanger 29 anddownstream of internal heat exchanger 29 and upstream of compressor 26.This allows a suction gas mass flow upstream of compressor 26 to bereduced far enough via second bypass 32 to avoid inadmissibly high finalcompression temperatures.

FIG. 5 shows a cooling device 36 which differs from the cooling deviceof FIG. 4 in that it has another cooling circuit 37. Other coolingcircuit 37 serves to cool a condenser 38 of a cooling circuit 39.Condenser 38 is realized as a cascade heat exchanger 40 in the case athand. Furthermore, cooling circuit 39 has another bypass 41 havinganother expansion element 42. Other bypass 41 is connected to coolingcircuit 39 downstream of compressor 26 and upstream of condenser 38 anddownstream of internal heat exchanger 29 and upstream of compressor 26.Thus, refrigerant not yet liquefied but compressed can flow back toupstream of compressor 26 via other bypass 41, whereby a suction gastemperature and/or a suction gas pressure of the refrigerant can becontrolled.

FIG. 6 shows a cooling device 30 having a cooling circuit 44 and anothercooling circuit 45 and, in particular, an internal heat exchanger 46 incooling circuit 44. In the case at hand, a heat exchanger 47 is disposedin a temperature-insulated test space of a test chamber (not shown).

1. A refrigerant for a cooling device having a cooling circuitcomprising at least one heat exchanger in which the refrigerantundergoes a phase transition, the refrigerant being a refrigerantmixture composed of a mass fraction of carbon dioxide (CO₂) and a massfraction of at least one other component, wherein the mass fraction ofcarbon dioxide in the refrigerant mixture is 10 to 50 mass percent,preferably 30 to 50 mass percent, the other component beingpentafluoroethane (C₂HF₅) and/or difluoromethane (CH₂F₂).
 2. Therefrigerant according to claim 1, wherein the mass fraction of carbondioxide in the refrigerant is 31 to 46 mass percent.
 3. The refrigerantaccording to claim 1, a mass fraction of carbon dioxide is 30 to 33 masspercent, and a mass fraction of pentafluoroethane is 70 to 67 masspercent.
 4. The refrigerant according to claim 1, wherein the massfraction of carbon dioxide is 44 to 48 mass percent, and a mass fractionof difluoromethane is 56 to 52 mass percent.
 5. The refrigerantaccording to claim 1, wherein the refrigerant mixture is a binarymixture.
 6. The refrigerant according to claim 1, wherein therefrigerant mixture has trifluoroethylene, 1,1-difluoroethene (C₂H₂F₂)or fluoroethene (C₂H₃F) as an additional component.
 7. The refrigerantaccording to claim 1, wherein a mass fraction of carbon dioxide is 33 to38 mass percent, a mass fraction of pentafluoroethane is 33.5 to 31 masspercent, and a mass fraction of difluoromethane is 33.5 to 31 masspercent.
 8. The refrigerant according to claim 1, wherein a massfraction of carbon dioxide is 33 to 38 mass percent and a mass fractionof refrigerant R410A or refrigerant R410B is 67 to 62 mass percent. 9.The refrigerant according to claim 1, wherein the refrigerant is azeotropic refrigerant and has a temperature glide of ≥10 K.
 10. Therefrigerant according to claim 1, wherein the refrigerant has a relativeCO2 equivalent of <2500 over 20 years.
 11. The refrigerant according toclaim 1, wherein the refrigerant is nonflammable.
 12. A test chamber forconditioning air, the test chamber comprising a test space which servesto receive test material and which can be sealed against an environmentand is temperature-insulated, and a temperature control device forcontrolling the temperature of the test space, a temperature in atemperature range of −60° C. to +180° C. being establishable within thetest space by means of the temperature control device, the temperaturecontrol device having a cooling device comprising a cooling circuit witha refrigerant according to claim 1, a heat exchanger, a compressor, acondenser and an expansion element.
 13. The test chamber according toclaim 12, wherein the cooling circuit has an internal heat exchanger,the internal heat exchanger being connected to a high-pressure side ofthe cooling circuit upstream of the expansion element and downstream ofthe condenser and to a low-pressure side of the cooling circuit upstreamof the compressor and downstream of the heat exchanger.
 14. The testchamber according to claim 12, wherein the heat exchanger is of such asize that the refrigerant can only partially evaporate in the heatexchanger.
 15. The test chamber according to claim 12, wherein thecondenser is realized as a cascade heat exchanger of another coolingcircuit of the cooling device.
 16. The test chamber according to claim12, wherein the temperature control device has a heating devicecomprising a heater and a heating heat exchanger in the test space. 17.The test chamber according to claim 12, wherein a first bypass having atleast one controllable second expansion element is realized in thecooling circuit, the first bypass being connected to the cooling circuitupstream of the internal heat exchanger and downstream of the condenser,the first bypass being realized as an additional controllable internalcooling.
 18. A use of a refrigerant consisting of a refrigerant mixturecomposed of a mass fraction of carbon dioxide (CO₂) of 10 to 50 masspercent, and a mass fraction of at least one other component, the othercomponent being pentafluoroethane (C₂HF₅) and/or difluoromethane(CH₂F₂), for conditioning air in a test space of a test chamber, thetest space serving to receive test material and being sealed against anenvironment and temperature-insulated, a cooling device of a temperaturecontrol device of the test chamber, which comprises a cooling circuitwith the refrigerant, a heat exchanger, a compressor, a condenser and anexpansion element, being used to establish a temperature in atemperature of −60° C. to +180° C. within the test space.
 19. The useaccording to claim 18, wherein an internal heat exchanger of the coolingcircuit, which is connected to a high-pressure side (17) of the coolingcircuit upstream of the expansion element and downstream of thecondenser and to a low-pressure side of the cooling circuit upstream ofthe compressor and downstream of the heat exchanger, is used to cool therefrigerant of the high-pressure side, the cooling of the refrigerant ofthe high-pressure side by means of the internal heat exchanger beingused to lower an evaporation temperature at the expansion element. 20.The use according to claim 18, wherein the refrigerant of thehigh-pressure side is cooled by the refrigerant of the low-pressure sideat a constant suction pressure on the low-pressure side by means of theinternal heat exchanger.